Certain processes for dewaxing petroleum distillates are well known. Dewaxing is required when highly paraffinic oils are to be used in products which must be mobile at low temperatures, e.g., lubricating oils, heating oils, and jet fuels. The higher molecular weight straight chain normal, substituted and slightly branched paraffins present in such oils are waxes that cause high pour points and high cloud points and haze points in the oils. If adequately low pour points are to be obtained, the waxes must be wholly or partially removed. In the past, various solvent removal techniques were employed to remove such waxes, such as propane dewaxing and MEK dewaxing; however, these have high operating costs, significant environmental impacts and produce oils which are inferior to catalytically-dewaxed oils. Catalytic dewaxing processes are more economical and remove the waxes by selectively isomerizing and cracking paraffinic components to produce lower molecular weight products, some of which may be removed by distillation.
Because of their selectivity, known dewaxing catalysts generally comprise an aluminosilicate zeolite having a pore size which admits the straight chain n-paraffins either alone or with only slightly branched chain paraffins, but which excludes more highly branched materials, larger cycloaliphatics and aromatics. Zeolites such as ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35 and ZSM-38 have been proposed for this purpose in dewaxing processes. Their use is described in U.S. Pat. Nos. 3,700,585; 3,894,938; 4,176,050; 4,181,598; 4,222,855; 4,229,282 and 4,247,388, the disclosures of which are incorporated herein by reference.
Since dewaxing processes of this kind function by means of cracking reactions, a number of useful products become degraded to lower molecular weight materials. For example, waxy paraffins may be cracked down to butane, propane, ethane and methane and so may the lighter n-paraffins which do not contribute to the waxy nature of the oil. Because these lighter products are generally of lower value than the higher molecular weight materials, it is desirable to limit the degree of cracking which takes place during a catalytic dewaxing process.
European Patent Application No. 225,053 discloses a process for producing lubricant oils by partially dewaxing a lubricant base stock by isomerization dewaxing followed by a selective dewaxing step. The isomerization dewaxing step is carried out using a large pore, high silica zeolite dewaxing catalyst such as high silica Y or zeolite beta which isomerizes the waxy components of the base stock to less waxy branched chain isoparaffins. The selective dewaxing step may be either a solvent, e.g., MEK dewaxing operation or a catalytic dewaxing, preferably using a highly shape zeolite such as ZSM-22 or ZSM-23.
U.S. Pat. No. 4,437,976 discloses a two-stage hydrocarbon dewaxing hydrotreating process wherein the pour point of a hydrocarbon charge stock boiling from 400.degree. F. to 1050.degree. F. is reduced by catalytically dewaxing the charge stock in the presence of a zeolite catalyst and subsequently subjecting at least the liquid portion thereof to hydrogenation in the presence of a hydrotreating catalyst comprising a hydrogenating component and a siliceous porous crystalline material from the class of ZSM-5, ZSM-11, ZSM-23 and ZSM-35 zeolites.
U.S. Pat. No. 4,575,416 to Chester et al. discloses a hydrodewaxing process with a first zeolitic catalyst having a Constraint Index not less than 1, a second catalytic component of specified characteristics and a hydrogenation component.
U.S. Pat. No. 5,149,421 teaches a dewaxing catalyst which provides superior selectivity with respect to the nature of the products obtained in a dewaxing process. By using an intermediate pore size silicoaluminophosphate molecular sieve catalyst in the dewaxing process, hydrocarbon oil feedstocks are effectively dewaxed and the products obtained are of higher molecular weight than those obtained using the other aluminosilicate zeolites. The products obtained from the dewaxing process have better viscosities and viscosity indexes at a given pour point as compared to the above-described prior art process using aluminosilicate zeolites.
Bright Stock, a lubricating oil base stock with a high boiling point (mostly &gt;1000.degree. F.), is difficult to dewax; a wax haze tends to remain while dewaxing a bright stock to the target pour point. This residual haze is indicated by a large spread between the haze point and the pour point of the dewaxed oil. A haze-pour point spread of less than 10-25.degree. C. is generally observed for most lube oils (with the haze point being higher than the pour point and higher than the cloud point). It is desirable to have the haze point be less than 10.degree. F. higher than the pour point. When dewaxing bright stock, pour-haze spreads of greater than 20.degree. C. are often encountered, at least in pilot plant work. Increasing dewaxing severity to reduce the haze-pour spread results in reduced yields, and often the haze point cannot be reduced enough, regardless of how low the pour point is reduced. It is desirable to identify a catalyst system which removes the wax haze during dewaxing to the target pour point (less than -9.degree. C. usually).
It would be advantageous to have an improved process for dewaxing and reducing the haze point of bright stocks. The present invention provides such a process.